The invention relates to high frequency magnetic components having high permeability cores and more specific high frequency inductors and the storing of energy therein.
For the purpose of this description the following words should be interpreted as follows: windingxe2x80x94one or more electrical conductors wound so as to be inductively coupled to a magnetic core.
Magnetic energy may be stored in magnetic components, i.e. inductors. The inductors may have cores made from high permeability material or low permeability. When storing magnetic energy in inductors having high permeability cores this requires at least one air-gap and/or several small air-gaps. The material in these high permeability cores may be made from e.g. ferrites, electrical steel, magnetic amorphous materials and magnetic nano-crystal materials. Penetration of fringing fields from the air-gap(s) will cause magnetic losses and also may result in damage to the device, such as hot spots etc.
One solution to the problem in the prior art is to use low permeability cores. The drawback with this solution is often expensive core(s) and problems with high current capacity.
The reason for using an air-gap/air-gaps is to avoid saturation of the core. The energy which is stored is proportional to the sizes of the air-gap. However, the magnetic field passes through he air-gap and the larger the air-gap is, the more this field will bulge outside from the air-gap volume as such, so called fringing fields. This will cause winding losses since the current will be distorted in the winding. In order to minimize these effects the winding may be distanced from the air-gap(s) e.g. by winding one or more layers f plastic foil closest to the air-gap(s) in order to separate the winding from the core. This is bad utilization of the winding window.
Another way of solving this problem is to divide the air-gap into several air-gaps or to cool the component.
Further, if the air-gap is larger, this may cause electromagnetic compatibility (EMC) problems. Ways to solve this problem in the known technique is to use copper strips in the winding instead of using copper wires or to make the air-gap smaller.
The present invention relates to a high frequency inductor of the type having a high permeability core with one or more air-gaps.
The present invention seeks to provide a high frequency inductor of the above type in which the drawbacks mentioned above are diminished.
In order to solve the above mentioned problems the inductor according to the invention exhibits a metal screen for reducing the penetration of the high frequency electromagnetic fringing field from the air-gap(s) into the winding. As a result the winding window can be fully utilized.
According to the invention copper wire may be used in the winding instead of copper strip, since the screen reduces the effect of the fringing field. The screen will protect the winding from hotspots and radiated EMI.
According to the invention the screen makes it possible to build the winding as layers of equal turns of wound wire connected in parallel. Typically an inductor conducts a DC/low frequency current with a superposed AC current. Due to the skin and the proximity effects the AC part of the current will be conducted by a conductor forming the inner winding layer. The inner layer can therefore be optimized for low AC losses. The DC part of the current will be shared by all the conductors in the winding by the principle of lowest electrical resistance. For lowest DC resistance the winding can therefore have as many parallel layers as the space allows, which implies high current capacity. The conductors in the DC part of the winding may also be wound all at the same time, giving a mix of those conductors in the outer layers.
Parallel winding layers without a screen have the drawback of circulating currents between the layers, caused by the high frequency fringing field from the air-gap(s).
Further according to the invention, by using winding layers connected in parallel a low total capacitance is obtained for the inductor.
A high frequency inductor according to the invention thus has a number of advantages. Among these are the possibility of storing more energy on account of the larger air-gap being shielded by the metal screen which is made possible without detrimental effects to the windings. Further, by providing separate AC and DC/low frequency parallel winding layers high current capacity and low capacitance are attained.